Conductive particles are widely used for anisotropic conductive materials such as anisotropic conductive paste, anisotropic conductive ink, an anisotropic conductive pressure sensitive adhesive/an anisotropic conductive adhesive, an anisotropic conductive film and an anisotropic conductive sheet by mixing them with a binder resin or a pressure sensitive adhesive/an adhesive and kneading.
These anisotropic conductive materials are used in the form that they are interposed between opposed boards or between opposed electrode terminals of electronic devices such as a liquid crystal display, a personal computer and a mobile telephone in order to electrically connect a board to another board or to electrically connect a small part such as a semiconductor device to a board.
As these conductive particles, there is disclosed the conductive particle in which a metal plating layer is formed as a conductive film on the surface of a nonconductive particle such as a resin particle having a uniform particle diameter and moderate strength (for example, in Patent Document 1).
In the conductive particle disclosed in Patent Document 1, a nickel plating coating is formed as a conductive film but a phosphorus concentration in the process of forming the nickel plating coating becomes low. In such a nickel plating coating having a low phosphorus concentration, a nickel plating coating having a crystal structure is formed. Since such a nickel plating coating is hard and has an inadequate property to follow the impact, there was a problem that the nickel plating coating might break and the adhesion between a base particle and the nickel plating coating was not good.
For these problems, a conductive particle having a nickel coating comprising a first layer in which a crystal grain aggregate is not found on the surface of the base particle and a second layer in which a crystal grain aggregate is oriented in the direction of thickness is disclosed in Patent Document 2. In this conductive particle, the first layer plays a role of enhancing the adhesion between the base particle and the nickel coating and further enhancing the impact resistance of the conductive particle and the second layer plays a role of improving the conductivity of the conductive particle.
However, it cannot be said that the production method disclosed in Patent Document 2 shows adequate performance in order to produce the conductive particle of a level required as electronic devices have been sharply progressing and developing in recent years, and therefore the need for producing a higher-performance conductive particle by a more strict production method has arisen.    Patent Document 1: Japanese Kokai Publication Sho-63-190204    Patent Document 2: Japanese Kokai Publication 2004-197160